1. Field of Invention
The invention relates to a controller for driving a synchronous motor while protecting an inverter from overheating.
2. Description of Related Art
Switching elements of inverters generally generate heat rapidly when synchronous motors are locked. The switching elements thus cannot be perfectly protected by simply measuring a temperature of the inverters with sensors and then limiting the current to be supplied to the inverters based on the measured temperature. The switching elements cannot be perfectly protected by using these steps because the difference between a measured temperature and an actual temperature of the switching elements is significantly large. Therefore, various techniques have been developed which prevent the overheating of switching elements by accurately estimating the temperature of the switching elements. JP-A-9-215388 (Paragraphs [0032]–[0041], FIG. 6), for example, describes a technique for limiting current supplied to the switching elements when a square integral of the current exceeds an allowed value. This technique is used because the integral of the current squared shows almost the same tendency as the increase in temperature of the switching elements.